The present invention relates to ferromagnetic thin-film structures exhibiting relatively large magnetoresistive characteristics and, more particularly, to such structures used for the storage and retrieval of digital data.
Many kinds of electronic systems make use of magnetic devices including both digital systems, such as memories, and analog systems such as magnetic field sensors. Digital data memories are used extensively in digital systems of many kinds including computers and computer systems components, and digital signal processing systems. Such memories can be advantageously based on the storage of digital symbols as alternative states of magnetization in magnetic materials provided in each memory storage cell, the result being memories which use less electrical power and do not lose information upon removals of such electrical power.
Such memory cells, and magnetic field sensors also, can often be advantageously fabricated using ferromagnetic thin-film materials, and are often based on magnetoresistive sensing of magnetic states, or magnetic conditions, therein. Such devices may be provided on a surface of a monolithic integrated circuit to provide convenient electrical interconnections between the device and the operating circuitry therefor.
Ferromagnetic thin-film memory cells, for instance, can be made very small and packed very closely together to achieve a significant density of information storage, particularly when so provided on the surface of a monolithic integrated circuit. In this situation, the magnetic environment can become quite complex with fields in any one memory cell affecting the film portions in neighboring memory cells. Also, small ferromagnetic film portions in a memory cell can lead to substantial demagnetization fields which can cause instabilities in the magnetization state desired in such a cell.
These magnetic effects between neighbors in an array of closely packed ferromagnetic thin-film memory cells can be ameliorated to a considerable extent by providing a memory cell based on an intermediate separating material having two major surfaces on each of which an anisotropic ferromagnetic memory thin-film is provided. Such an arrangement provides significant xe2x80x9cflux closure,xe2x80x9d i.e. a more closely confined magnetic flux path, to thereby confine the magnetic field arising in the cell to affecting primarily just that cell. This result is considerably enhanced by choosing the separating material in the ferromagnetic thin-film memory cells to each be sufficiently thin. Similar xe2x80x9csandwichxe2x80x9d structures are also used in magnetic sensors.
In the recent past, reducing the thicknesses of the ferromagnetic thin-films and the intermediate layers in extended xe2x80x9csandwichxe2x80x9d structures, and adding possibly alternating ones of such films and layers, i.e. superlattices, have been shown to lead to a xe2x80x9cgiant magnetoresistive effectxe2x80x9d being present in some circumstances. This effect yields a magnetoresistive response which can be in the range of up to an order of magnitude or more greater than that due to the well known anisotropic magnetoresistive response.
In the ordinary anisotropic magnetoresistive response, varying the difference occurring between the direction of the magnetization vector in a ferromagnetic thin-film and the direction of sensing currents passed through that film leads to varying effective electrical resistance in the film in the direction of the current. The maximum electrical resistance occurs when the magnetization vector in the field and the current direction therein are parallel to one another, while the minimum resistance occurs when they are perpendicular to one another. The total electrical resistance in such a magnetoresistive ferromagnetic film can be shown to be given by a constant value, representing the minimum resistance, plus an additional value depending on the angle between the current direction in the film and the magnetization vector therein. This additional resistance has a magnitude characteristic that follows the square of the cosine of that angle.
Operating magnetic fields imposed externally can be used to vary the angle of the magnetization vector in such a film portion with respect to the easy axis of that film. Such an axis comes about in the film because of an anisotropy therein typically resulting from depositing the film during fabrication in the presence of an external magnetic field oriented in the plane of the film along the direction desired for the easy axis in the resulting film. During subsequent operation of the device having this resulting film, such operational magnetic fields imposed externally can be used to vary the angle to such an extent as to cause switching of the film magnetization vector between two stable states which occur for the magnetization being oriented in opposite directions along the film""s easy axis. The state of the magnetization vector in such a film can be measured, or sensed, by the change in resistance encountered by current directed through this film portion. This arrangement has provided the basis for a ferromagnetic, magnetoresistive anisotropic thin-film to serve as a memory cell.
In contrast to this arrangement, the resistance in the plane of a ferromagnetic thin-film is isotropic for the giant magnetoresistive effect rather than depending on the direction of the sensing current therethrough as for the anisotropic magnetoresistive effect. The giant magnetoresistive effect involves a change in the electrical resistance of the structure thought to come about from the passage of conduction electrons between the ferromagnetic layers in the xe2x80x9csandwichxe2x80x9d structure, or superlattice structure, through the separating nonmagnetic layers with the resulting scattering occurring at the layer interfaces, and in the ferromagnetic layers, being dependent on the electron spins. The magnetization dependent component of the resistance in connection with this effect varies as the sine of the absolute value of half the angle between the magnetization vectors in the ferromagnetic thin-films provided on either side of an intermediate nonmagnetic layer. The electrical resistance in the giant magnetoresistance effect through the xe2x80x9csandwichxe2x80x9d or superlattice structure is lower if the magnetizations in the separated ferromagnetic thin-films are parallel and oriented in the same direction than it is if these magnetizations are antiparallel, i.e. oriented in opposing or partially opposing directions. Further, the anisotropic magnetoresistive effect in very thin films is considerably reduced from the bulk values therefor in thicker films due to surface scattering, whereas a significant giant magnetoresistive effect is obtained only in very thin films. Nevertheless, the anisotropic magnetoresistive effect remains present in the films used in giant magnetoresistive effect structures.
As indicated above, the giant magnetoresistive effect can be increased by adding further alternate intermediate nonmagnetic and ferromagnetic thin-film layers to extend a xe2x80x9csandwichxe2x80x9d structure into a stacked structure, i.e. a superlattice structure. The giant magnetoresistive effect is sometimes called the xe2x80x9cspin valve effectxe2x80x9d in view of the explanation that a larger fraction of conduction electrons are allowed to move more freely from one ferromagnetic thin-film layer to another if the magnetizations in those layers are parallel than if they are antiparallel or partially antiparallel to thereby result in the magnetization states of the layers acting as sort of a xe2x80x9cvalve.xe2x80x9d
Thus, a digital data memory cell based on the use of structures exhibiting the giant magnetoresistive effect is attractive as compared to structures based on use of an anisotropic magnetoresistive effect because of the larger signals obtainable in information retrieval operations with respect to such cells. Such larger magnitude signals are easier to detect without error in the presence of noise thereby leading to less critical requirements on the retrieval operation circuitry.
Memory cell structures suitable for permitting the storing and retaining of a digital bit of information, and for permitting retrieving same therefrom has been demonstrated based on a multiple layer xe2x80x9csandwichxe2x80x9d construction in a rectangular solid. This cell has a pair of ferromagnetic layers of equal thickness and area separated by a conductive nonmagnetic layer of the same shape and area parallel to the ferromagnetic layers but of smaller thickness. These ferromagnetic layers are each a composite layer formed of two strata each of a different magnetic material, there being a relatively thin ferromagnetic stratum in each of the composite layers adjacent the nonmagnetic layer and a thicker ferromagnetic stratum in each of the composite layers adjacent the thin ferromagnetic stratum therein. The ferromagnetic material of the thick stratum in one of the composite layers is the same as that in the thin stratum in the other composite layer, and the ferromagnetic material of the thin stratum in the first composite layer is the same as the ferromagnetic material in the thick stratum of the second composite layer. Each of the composite layers is fabricated in the presence of a magnetic field so as to result in having an easy axis parallel to the long sides of the rectangular solid. The dimensions of cell structures as part of development efforts are in many instances relatively large for development purposes.
One such memory cell structure has a pair of ferromagnetic layers of matching geometries but different magnetic materials in the strata therein to result in one such layer having effectively a greater saturation magnetization and a greater anisotropy field than the other to result in different coercivities in each. In addition, the structure results in a coupling of the magnetization between the two ferromagnetic layers therein due to exchange coupling between them leading to the magnetizations in each paralleling one another in the absence of any applied magnetic fields. As a result, the electrical resistance of the cell along its length versus applied magnetic fields in either direction parallel thereto is represented by two characteristics depending on the magnetization history of the cell. Each of these characteristics exhibits a peak in this resistance for applied longitudinal fields having absolute values that are somewhat greater than zero, one of these characteristics exhibiting its peak for positive applied longitudinal fields and the other characteristic exhibiting its peak for negative applied longitudinal fields. The characteristic followed by the resistance of the cell for relatively small applied longitudinal fields depends on which direction the magnetization is oriented along the easy axis for the one of the two ferromagnetic layers having the larger coercivity. Thus, by setting the magnetization of the layer with the higher coercivity, a bit of digital information can be stored and retained, and the value of that bit can be retrieved without affecting this retention through a determination of which characteristic the resistance follows for a relatively small applied longitudinal field.
Such memory cell behavior for this structure can be modeled by assuming that the ferromagnetic layers therein are each a single magnetic domain so that positioning of the magnetization vectors in the ferromagnetic layers is based on coherent rotation, and that uniaxial anisotropies characterize those layers. The angles of the magnetization vectors in the two ferromagnetic layers with respect to the easy axis in those layers are then found by minimizing the magnetic energy of these anisotropies summed with that due to the applied fields and to exchange coupling. That total energy per unit volume is then                               E          Tot                =                  xe2x80x83                ⁢                              E            1                    +                      E            2                    +                      E            12                                                  =                  xe2x80x83                ⁢                                            K              u1                        ⁢                          sin              2                        ⁢                          θ              1                                -                                    M              s1                        ⁢            H            ⁢                          xe2x80x83                        ⁢                          cos              ⁡                              (                                  Ψ                  -                                      θ                    1                                                  )                                              +                                                  xe2x80x83                ⁢                                            K              u2                        ⁢                          sin              2                        ⁢                          θ              2                                -                                    M              s2                        ⁢            H            ⁢                          xe2x80x83                        ⁢                          cos              ⁡                              (                                  Ψ                  -                                      θ                    2                                                  )                                              +                                                  xe2x80x83                ⁢                              A            12                    ⁢                                    cos              ⁡                              (                                                      θ                    1                                    -                                      θ                    2                                                  )                                      .                              
Here, Ku1, and Ku2 are anisotropy constants, A12 is the exchange constant, Ms1 and Ms2 are the magnetization saturation values, and H is the externally applied field. As indicated above, once the magnetization vectors have taken an angular position with respect to the easy axis of the corresponding layer at a minimum in the above indicated energy, the effective resistance between the ends of the memory cell structure is determined by the net angle between the magnetization vectors in each of these layers.
Because of the assumption of single domain behavior in the ferromagnetic layers, the above equation would seemingly be expected to improve its approximation of the assistant total magnetic energy as the length and width of that memory cell structure decreased toward having submicron dimensions. However, this mode of operation described for providing the two magnetoresistive characteristics based on the history of the layer magnetizations, in depending on the differing anisotropy fields in the two ferromagnetic layers because of the differing materials used therein, becomes less and less reliable as these dimensions decrease. This appears to occur because decreasing the cell dimensions gives rise to larger and larger demagnetizing fields in the two ferromagnetic layers which, at some point, overwhelm the effects of the anisotropy fields so that the above described behavior no longer occurs as described. In addition, the magnetizations of the two ferromagnetic layers rotate together under the influences of externally applied fields at angles with respect to the corresponding easy axis at angular magnitudes much more nearly equal to one another because of the increasing demagnetization fields in these layers as the dimensions thereof decrease. As a result, these ferromagnetic layers are less and less able to have the magnetizations thereof switch directions of orientation independently of one another as the dimensions thereof decrease so that the structure they are in becomes less able to provide the above described memory function in relying on only these ferromagnetic layer anisotropy differences.
An alternative memory cell structure which is more suited to submicron dimensions is a cell of the kind described above exhibiting xe2x80x9cgiant magnetoresistive effectxe2x80x9d but which has the two composite ferromagnetic layers formed of different thicknesses in the thick strata therein. Thus, the thick strata in one might be on the order of 40 xc3x85 while that of the other might be on the order of 55 xc3x85 as an example. In this structure, reducing the size to submicron dimensions uses the shape anisotropy introduced by this thickness difference to provide different switching thresholds for each of the ferromagnetic composite layers in response to externally applied operating magnetic fields. The shape anisotropy leads to the effect of the demagnetizing field of one layer affecting the switching threshold of the other after the former layer has switched its magnetization direction. As a result, the thicker ferromagnetic layer has a magnetization which is fixed in orientation for externally applied operating magnetic fields that are just sufficient to switch the thinner ferromagnetic composite layer but not great enough to switch the magnetization of the thicker ferromagnetic composite layer. In effect, the demagnetizing fields as the device becomes sufficiently small dominate the anisotropy fields that result from the deposition of the ferromagnetic layers in the presence of a magnetic field.
In the absence of externally applied operating magnetic field, the two composite ferromagnetic layers have the magnetizations therein pointing in opposite directions, i.e. they are antiparallel to one another, to result in the structure as a whole having relatively small cell demagnetizing fields and small external stray fields to affect the nearby memory cells. The direction of magnetization in the thicker ferromagnetic composite layer is used to store the digital information which can only be changed in direction by externally applied fields great enough to switch magnetization directions in both composite ferromagnetic layers. That is, storing new information in the cell requires that the thicker ferromagnetic layer be capable of having the magnetization direction therein switched to be in accord with the incoming digital data.
Retrieving information from such a memory cell is accomplished by switching the magnetization direction of the thinner ferromagnetic composite layer only as a basis for determining in which direction relative to the thinner layer is the magnetization oriented in the thicker layer. Typically, both such storing and retrieving has meant that there needs to be a pair of external conductors which can coincidentally supply current to result in a field large enough to switch the magnetization of the thicker ferromagnetic composite layer, but with that current in either conductor alone being able to generate fields only sufficient to switch the threshold of the thinner ferromagnetic layer. In some situations, only a single external conductor need be provided for this purpose because the sense current used in retrieving information from the memory cell can provide the coincident current needed with the current in the external conductor to switch the magnetization direction of the thicker ferromagnetic layer. Such a memory cell is described in U.S. Pat. 5,996,322 to A. Pohm and B. Everitt entitled xe2x80x9cGiant Magnetoresistive Effect Memory Cellxe2x80x9d assigned to the same assignee as the present application and which is hereby incorporated herein.
Alternatively, a digital data bit storage and retrieval memory cell suited for fabrication with submicron dimensions can be fabricated that provides rapid retrievals of bit data stored therein and low power dissipation by substituting an electrical insulator for the conductor in the nonmagnetic layer. This memory cell can be fabricated using ferromagnetic thin-film materials of similar or different kinds in each of the magnetic memory films used in a xe2x80x9csandwichxe2x80x9d structure on eitherside of an intermediate nonmagnetic layer which ferromagnetic films may be composite films, but this intermediate nonmagnetic layer conducts electrical current therethrough based primarily on a quantum electrodynamic effect xe2x80x9ctunnelingxe2x80x9d current.
This xe2x80x9ctunnelingxe2x80x9d current has a magnitude dependence on the angle between the magnetization vectors in each of the ferromagnetic layers on either side of the intermediate layer due to the transmission barrier provided by this intermediate layer depending on the degree of matching of the spin polarizations of the electrons tunneling therethrough with the spin polarizations of the conduction electrons in the ferromagnetic layers, the latter being set by the layer magnetization directions to provide a xe2x80x9cmagnetic valve effectxe2x80x9d. Such an effect results in an effective resistance or conductance characterizing this intermediate layer with respect to the xe2x80x9ctunnelingxe2x80x9d current therethrough. In addition, shape anisotropy can be used in such a cell to provide different magnetization switching thresholds in the two ferromagnetic layers by forming one of the ferromagnetic layers to be thicker than the other, especially in those situations in which the same material is used for each of the ferromagnetic layers on either side of the intermediate layer. Such devices may be provided on a surface of a monolithic integrated circuit to thereby allow providing convenient electrical connections between each such memory cell device and the operating circuitry therefor.
A xe2x80x9csandwichxe2x80x9d structure for such a memory cell, based on having an intermediate thin layer of a nonmagnetic, dielectric separating material with two major surfaces on each of which a anisotropic ferromagnetic thin-film is positioned, exhibits the xe2x80x9cmagnetic valve effectxe2x80x9d if the materials for the ferromagnetic thin-films and the intermediate layers are properly selected and have sufficiently small thicknesses. The resulting xe2x80x9cmagnetic valve effectxe2x80x9d can yield a response which can be several times in magnitude greater than that due to the xe2x80x9cgiant magnetoresistive effectxe2x80x9d in a similarsized cell structure. Such a memory cell is described in U.S. Pat. No. 5,996,322 indicated above, and in an earlier filed co-pending application by J. Daughton, B. Everitt and A. Pohm entitled xe2x80x9cSpin Dependent Tunneling memoryxe2x80x9d having Ser. No. 09/435,598 assigned to the same assignee as the present application and which is hereby incorporated herein.
Further alternatives to each of the foregoing kinds of memory cell structures are made available by the use therein of additional structure to provide different switching thresholds for each of he ferromagnetic composite layers in response to externally applied operating magnetic fields rather than (or in addition to) ferromagnetic layer thickness differences or material differences. The greater stability of one ferromagnetic layer to externally applied operating magnetic fields than the other as a basis for storing information can be provided by using an added antiferromagnetic layer, or composite layer including an antiferromagnetic layer, to xe2x80x9cpinxe2x80x9d the magnetization direction of one of the ferromagnetic layers in a selected direction while leaving the remaining ferromagnetic layer or layer relatively unaffected with respect to such stability against externally applied operating magnetic fields. Such ferromagnetic layers and antiferromagnetic layers each are coupled moment material films in which magnetic moments of adjacent atoms, ions or molecules are coupled to one another to maintain some alignment thereof below a critical temperature, but antiferromagnetic films have such coupled moments balanced in opposite directions to have no net magnetic moment.
Thus, a further antiferromagnetic material xe2x80x9cpinningxe2x80x9d layer is deposited immediately adjacent the ferromagnetic layer selected for xe2x80x9cpinningxe2x80x9d to strongly set the magnetization direction of that layer. Such an antiferromagnetic layer through exchange coupling to the adjacent ferromagnetic layer strongly fixes the selected direction of magnetization of that ferromagnetic layer. An IrMn antiferromagnetic pinning layer of 20% iridium and 80% manganese can be sputterdeposited adjacent the selected ferromagnetic layer to a thickness of 100 xc3x85 in the presence of a magnetization axis determination magnetic field aligned with the fields used in selecting the direction of magnetization of the ferromagnetic layer to accomplish the pinning thereof. Alternatively, the antiferromagnetic pinning layer can be formed using FeMn, NiMn, PtMn, PtRhMn or PtRuMn or some other antiferromagnetic materials.
All of these foregoing kinds of memory cells are desired to be shrunk in size to the extent possible to increase the cell density across the area of the substrate supporting them, such as an electrically insulative layer substrate formed in a monolithic integrated circuit containing the operating circuitry for operating such a memory. However, as the design widths of these cells are shrunk to small fractions of a micron, the subsequently fabricated cells have a correspondingly greater reduction in relative total cell volume.
Memory cells without xe2x80x9cpinningxe2x80x9d layers therein have differences in switching thresholds for each of the included ferromagnetic composite layers in response to externally applied operating magnetic fields determined primarily by the difference in the transverse demagnetizing fields of these two ferromagnetic layers and the volumes thereof so that this threshold difference decreases with decreasing volume. This threshold difference is, of course, effectively reduced during storage and retrieval operations of a memory array if a current is introduced either as a word line current through the word line adjacent a memory cell or a sense current through the memory cell as either provides magnetic fields directed to cause ferromagnetic layer magnetization direction switching. Even though both such currents are intended to be required to occur with respect to a cell for the magnetization direction of the more stable ferromagnetic layer therein to switch direction, the provision of one such current in a cell with relatively small switching threshold due to small cell volume may allow thermal fluctuations in the cell to provide enough cell energy to affect the threshold so as to allow its being exceeded in these circumstances. Cells with a xe2x80x9cpinnedxe2x80x9d layer have improved thermal stability allowing further scaling to even smaller sizes but will at some point also become susceptible to thermal fluctuations.
Thus, a typical memory cell without a xe2x80x9cpinnedxe2x80x9d layer, having a word line current applied in the word line adjacent thereto that provides a corresponding magnetic field equal to one half of the cell thicker layer anisotropy field, has the added energy required to result in switching reduced to a quarter of the value needed in the absence of such word line current, i.e. to a quarter of xc2xd*Hk*Ms*V. Here, Hk is the anisotropy field, Ms is the magnetic saturation and V is the volume. Of course, if such cells with a word line current provided are stable against thermal fluctuations, those cells in the array without such currents present should also be stable.
In operating commercial semiconductor devices, junction temperatures are usually limited to 125xc2x0 C. or less because of increased structural failures in such devices and in the interconnections therebetween at elevated temperatures. Thus, such a temperature is a reasonable upper operating temperature limit for such cells. This allows a maximum operating temperature rise of 100xc2x0 C. from an assumed ambient temperature of 25xc2x0 C.
The failure rate for semiconductor devices due to structural failures therein has been observed to be on the order of 10xe2x88x924 events per year. Thus, a conservative requirement for thermally induced errors is to have them be relatively insignificant by requiring them to be an order of magnitude less frequent, or 10xe2x88x925 events per year. Although such thermally induced errors can be corrected with error correction circuitry, the error rate increases very rapidly as the average energy of thermal fluctuations approaches the energy required to switch the magnetization states of memory cell devices.
Memory cells considered as single magnetic domains (a reasonable assumption for the very small cells involved) have been determined to exhibit spatial oscillations in their shapes as a result of external stimuli such as thermal fluctuations in two normal modes, which oscillations can lead to corresponding oscillatory changes in the switching threshold values thereof. The associated decay times for such oscillatory behaviors have been observed to typically be around 0.5 and 1.0 ns for the two modes. Choosing the mode with the shorter decay time to provide the greater number of oscillations, the inverse of the associated decay time provides a measure of the number of oscillatory threshold minimums (and maximums) occurring in that mode per second. Multiplying this number of minimums per second by the number of memory cells under the influence of magnetic fields established by currents through a word line, the fraction of time that a word line has currents flowing therein during memory operation, and the number of seconds in a year provides an approximation of the number of oscillatory threshold minimums occurring in a year.
The energy of thermal fluctuations stimulating memory cells in a memory device is randomly distributed over an energy range which, from thermodynamics, can in many circumstances be represented by the well known Boltzmann probability distribution involving the average energy of the fluctuations. The difference between this average energy and the cell switching energy required after a word current is initiated in the word line adjacent to the cell forms an energy well, Ew, which must be maintained to meet the desired thermally induced error rate, or
10xe2x88x925=({fraction (1/0.5)}*10xe2x88x929)*(500)*(⅓)*(3600*24*365)*exe2x88x92Ew/kT.
Here, {fraction (1/0.5)}*10xe2x88x929 is the inverse of the relaxation time, 500 is a typical number of memory cells affected by the magnetic fields arising from a word line, ⅓ is the fraction of operating time current is introduced in a word line, 3600*24*365 is the number of seconds in a year, and exe2x88x92Ew/kT is the Boltzmann distribution factor with T being the absolute temperature and k being Boltzmann""s constant.
From the foregoing equation, an energy well of 55 kT is needed for thermal stability after current is introduced in a word without sense currents being initiated in the cells under the influence of that word line, or just less than 2 electron volts. To meet this requirement for a memory cell on the order of 0.1 xcexcm wide by 0.4 xcexcm long, the shape anisotropy of the thicker and thinner ferromagnetic layers must be at least 500 Oe and 300 Oe respectively. Since the width and length for this cell are already determined, these anisotropy fields must be provided by properly choosing the thicknesses of these layers requiring 40 to 50 xc3x85 of thickness for the thicker layer and 20 to 25 xc3x85 of thickness for the thinner layer. The magnetic fields necessary to reach the layer thresholds to cause switching of the thicker layer magnetization direction for memory cells of smaller and smaller lengths and widths begins to require currents through such cells and associated word lines of magnitudes that result in current densities sufficient to cause significant electromigration of the conductive materials and operating temperature rises which will alter device behavior and structure thereby leading to a limit of some minimum size for these cells. Thus, there is a desire to find another method of operating such cells to store information therein.
The present invention provides a ferromagnetic thin-film based digital memory having in a bit structure a coupled moment material film in which magnetic moments of adjacent atoms, ions or molecules are coupled to one another to maintain some alignment thereof below a critical temperature above which such alignment is not maintained, and also having a plurality of word line structures each located across from the coupled moment material film in a corresponding one of the bit structures, these bit structures being sufficiently thermally isolated to allow currents in the adjacent word lines and/or the bit structure to heat the bit structure to approach the critical temperature. Currents can be supplied coincidently in the adjacent word line and the bit structure, or instead in a further conductor also adjacent the bit structure rather than in that structure, to cause such a temperature rise, and then reduced to cool the bit structure while supplying a magnetic field during the cooling to select the direction of magnetization to be maintained thereafter until new data is next stored therein.